1. Field of the Invention
The present invention relates to a variable capacitor in which spacing between a fixed electrode and a movable electrode facing each other varies depending on voltage applied.
2. Description of the Related Art
A variable capacitor included in an oscillator circuit or a modulator circuit of, for example, a mobile phone, can change a capacitance so that the circuit yields a desired output. Such a capacitor has recently been achieved using microelectromechanical systems (MEMS) technology. Specifically, the MEMS variable capacitor includes a fixed electrode, a movable electrode facing the fixed electrode with a predetermined spacing therebetween, a beam supporting the movable electrode displaceably relative to the fixed electrode, and a drive electrode for applying a drive voltage between the fixed electrode and the movable electrode. The MEMS variable capacitor is configured such that a force (e.g., electrostatic force, magnetic force, or piezoelectric force) generated between the fixed electrode and the movable electrode upon application of the drive voltage causes the movable electrode to be displaced, the spacing between the fixed electrode and the movable electrode facing each other accordingly varies, and the capacitance therefore changes.
Related-art variable capacitors are of DC-drive type in which a DC drive voltage is applied between the fixed electrode and the movable electrode, as disclosed in Japanese Unexamined Patent Application Publication No. 9-55337, for example. Accordingly, a high drive voltage is necessary to achieve a large change in capacitance of the variable capacitor. Disadvantageously, this leads to increased power consumption. Additionally, a step-up circuit for generating such a high voltage is also required. Unfortunately, an apparatus is inevitably increased in size. Since portable apparatuses are, however, always required to have small size and low profile, including such a variable capacitor is a disadvantage in terms of achieving small size and low profile. Furthermore, if a variable capacitor element is configured to include a dielectric layer deposited on an upper surface or lower surface of the fixed electrode or the movable electrode, continuous application of a high voltage between the fixed electrode and the movable electrode may cause the dielectric layer to be charged such that the fixed electrode will be attached to the movable electrode. Disadvantageously, the variable capacitor element may fail to function.
To overcome the above-described disadvantages caused by DC driving, an AC drive voltage may be used instead of a DC drive voltage. In the use of the AC drive voltage, since the drive voltage to be applied between the fixed electrode and the movable electrode fluctuates, the movable electrode vibrates in directions close to and away from the fixed electrode. This causes ripple in the capacitance of the variable capacitor. Unfortunately, such a capacitor is difficult to use as a circuit element.